In optical communications or other such communication systems, achievement of an increase in capacity and reduction in energy consumption per unit of communication capacity have become pressing issues. Of those, for the achievement of an increase in capacity, a reception value is subjected to multi-level discrimination, and calculation of a likelihood and error correction are performed based on a result of the discrimination, to thereby achieve a communication channel with a large capacity and high reliability.
In regard to the likelihood to be calculated, it is common to obtain, with reference to the reception value, a minimum value of errors (Euclidean distances) from all transmission candidate values of a pattern in which a transmission bit is 1 and a minimum value of errors from all transmission candidate values of a pattern in which a transmission bit is 0, and to set a difference therebetween as a log likelihood ratio (LLR).
When a circuit that operates in real time is constructed, a real-time property is ensured by implementing likelihood generation circuits in parallel with one another. As a result, there is a problem that a circuit scale increases in accordance with a parallel number thereof. An increase in circuit scale causes an increase in energy consumption. That is, as a result of increasing the parallel number in order to achieve an increase in capacity, there arise a problem that the energy consumption of a communication system increases and a problem that heat dissipation becomes difficult.
In view of the foregoing, in order to achieve both the increase in communication capacity and the reduction in energy consumption, there is a related art that employs the following method. That is, there is a related art configured to select a scheme at a time of circuit designing so as to use a scheme excellent in performance but high in energy consumption for a part liable to cause degradation in signal quality and to use a scheme inferior in performance but low in energy consumption for a part that exerts little influence on the signal quality (see, for example, Patent Literature 1).
As another method, in order to achieve an increase in capacity, there is a related art configured to estimate communication channel quality and to dynamically select an optimum error correction algorithm based on a result of the estimation (see, for example, Patent Literature 2).
A reception frequency of a reception value is not uniform and has variations. In addition, a likelihood is uniquely determined when a reception value is obtained. Therefore, in the same manner as in the reception frequency of the reception value, variations occur in a distribution of the calculated likelihood. It is also common that, as the number of kinds of values that can be taken by the reception value becomes smaller, the likelihood calculation circuit becomes simpler and lower in energy consumption.
Therefore, it is possible to lower average energy consumption required for likelihood calculation by providing a likelihood calculation scheme dedicated to a frequently received value and a likelihood calculation scheme for handling all values that can be taken by the reception value, and selecting a likelihood calculation scheme suitable for each reception value.